1. Field of the Invention
The present invention relates to a radiographic image detector and a gain setting method for the radiographic image detector.
2. Description of the Related Art
In the medical field, radiography systems utilizing radioactive rays such as x-rays for imaging are known. An x-ray radiography system includes an x-ray projector having an x-ray source for radiating x-rays, and radiographic equipment that receives x-rays after having been projected from the x-ray projector toward a subject and penetrating through the subject, thereby to acquire a radiograph or x-ray image that represents information on the subject. As the radiographic equipment, an x-ray image detector using a flat panel detector (FPD) in place of conventional x-ray film or an imaging plate (IP) has recently been developed and used in practice, which can output digital data of an acquired x-ray image, as disclosed in JPA 2009-219538.
As described in this prior art, the FPD includes a detection panel having an imaging area in which a large number of pixels are arranged in a matrix to accumulate signal charges corresponding to incident amounts of x-rays on the respective pixels, and a signal processing circuit for reading out the signal charges as digital image data. The signal processing circuit has a voltage output circuit for outputting the signal charges accumulated in the pixels as an analog voltage signal, and an A/D converter for converting the voltage signal to digital image data.
Through the A/D conversion, the voltage signal representing the amounts of charges accumulated in the pixels are converted to pixel levels that represent tonal levels in density gradation of the x-ray image. Each pixel level is expressed by a digital value within a dynamic range that is defined by the bit number of the A/D converter. The above-mentioned prior art discloses an FPD that can adjust amplification or gain of amplifiers for the voltage signal according to the dose of x-rays onto the FPD in order to make good use of the dynamic range of the A/D converter.
The x-ray dose on the FPD varies depending upon exposure conditions determined by the target site of radiography and other factors. Because the amounts of signal charges accumulated in the pixels increase in proportion to the radiation dose, the density of x-ray image will increase with the dose. In the above prior art, the dosed amount of x-rays is measured, and the gain is set low for high-dose images or high for low-dose images, so as to make effective use of the dynamic range of the A/D converter.
Applying a high gain to low-dose images boosts up the pixel levels so that low pixel levels representative of lower image densities may be tunable into the dynamic range of the A/D converter. By contrast, applying a low gain to high-dose images prevents pixel level saturation in the high density range so that the entire pixel levels are tuned into the dynamic range of the A/D converter. Thus, high-quality x-ray images may be provided with adequate density gradation.
The prior art suggests two kinds of gain setting methods: in the first method the gain is set at a single value and commonly applied to every pixel while a frame of image is being read out from the pixels, whereas in the second method, different gain values are determined for different subareas of one image frame, such as a central area and a peripheral area, according to variable doses measured in the respective subareas.
In the first method applying the same gain to all pixels in one image frame, the gain is determined by the total amount of radiation dose on the entire imaging area. In that case, if the total dose is the same, it is impossible to discriminate between a high-contrast image, which contains high density areas and low density areas with great density differences, and a low-contrast image having intermediate densities in the whole area. Accordingly, if the total dose is the same, the same gain will be applied to these images regardless of whether the contrast in the individual image is high or low. As a result, it sometimes happens that the subsequent image does not adequately reproduce the density gradation.
On the other hand, according to the second method of the prior art, applied gain may differ from one subarea to another within an image frame according to the respective densities of the subareas. Therefore, tone reproduction of the subsequent image will be improved as compared to the first method. However, as described in the prior art (in paragraph 0116), the second method needs compensation process for the differentiated gains between the subareas of the image frame in addition to other image rendering or correcting processes after the A/D conversion, such as offset correction and sensitivity correction. Therefore, the second method will complicate the image correcting processes after the A/D conversion.